"Il parle des robes."
de and des are fairly easy to distinguish between even with the duo robot.
Go to Google Translate for practice at hearing the sounds of simple words like de and des.
Google Translate should not be taken as a final authority on how to pronounce French. But it does help you learn what you should be listening for.
Good point. I believe it is more literally "of", or "of the" or "of some". "He speaks of dresses" would be an old-fashioned way of saying "He speaks about dresses". "Les robes" is "the dresses", and "of" would often be transleted by "de". But when "de" comes before "les", the two are combined into "des" - you don't say "de les".
Another rule of thumb that I ran across is: use de (of) when speaking in general as in he speaks of dresses .
When speaking in particular, he speaks of the dresses , use des . Here he is speaking of all examples of something in particular. of the/de les is contracted to read des.
On the basis of what I have read the only way to be absolutely sure which is correct is to be absolutely sure of the context. Even then it can be dicey.
Given Duo's use of fragments of speech that make it pretty hard to establish the exact context, many times either answer is correct even if Duo says it's not (on the basis of the information provided). Of course, Duo isn't trying to teach undoing linguistic knots but simply exposing students to different uses.
Whatever Duo's purpose they have certainly have caused me to read about du.de, les, des, etc. and speculate about the real meaning of some, about, of etc. more than I ever thought possible when I started French a month ago.
If it's any consolation to anyone it seems that these particular issues are regarded as difficult even for advanced students of French.
The pronunciation of the article in front of the word is what hints at the presence or absence of the 's'. - "des robes" sounds like "deh robes", while "de robe" has a shorter 'e' sound, almost like what you would say if you were sounding out the alphabet and just saying "d...". - 'robe' and 'robes' are pronounced the same, but 'des', 'de' (and 'du'), are all pronounced differently. It might be hard to hear at first, but your brain can get used to it.
Sometimes you pronounce the 's', sometimes you don't: http://french.about.com/library/pronunciation/bl-liaisons.htm
As mentioned previously in this thread des (de les) means of the when used in this context. He is not speaking of dresses (in general) but rather of the dresses (a particular group of dresses).
We know this because the writer put it as de les which he contracted to des which means literally of the.
Des also has the meaning of some but that usage wouldn't make sense in this sentence.
La is singular and so couldn't be used in connection with robes.